It says it has ordered tests into whether the horse meat has traces of a banned equine drug, phenylbutazone, or 'bute', which is harmful to humans.

Findus, Tesco and Aldi all withdrew a variety of frozen beef products from sale yesterday following new information from a French supplier, Comigel.

Tesco and Aldi have said the frozen lasagne and spaghetti Bolognese do not conform to specification but have not specified the problem.

There are growing suspicions that they, too, contain horse meat.

Tonight, the FSA said in a statement: "As part of its ongoing investigation into mislabelled meat, the Food Standards Agency has confirmed that the meat content of beef lasagne products recalled by Findus has tested positive for more than 60 per cent horse meat."

It added: "We have no evidence to suggest that this is a food safety risk. However, the FSA has ordered Findus to test the lasagne for the veterinary drug phenylbutazone, or 'bute'.

"Animals treated with phenylbutazone are not allowed to enter the food chain as it may pose a risk to human health.

The Findus beef lasagne was distributed to the main UK supermarkets and smaller convenience stores.

The FSA said: "Findus has already begun a full recall of these products. People who have bought any Findus beef lasagne products are advised not to eat them and return them to the shop they bought them from."

Ten million budget beefburgers were last month withdrawn from sale at five supermarket chains - including Tesco and Aldi - after tests by the Food Safety Authority of Ireland found the presence of horsemeat.

More meat in Ireland and Northern Ireland has been found to have horsemeat amounting , respectively, to 75 and 80 per cent of the stated beef content.

Mary Creagh MP, Labour's Shadow Environment Secretary, criticised the UK authorities for responding slowly to the concerns that the British meat supply chain is similarly contaminated.

She said: "The Irish Government has called in the police and fraud experts to investigate the horse meat fraud, yet complacent British ministers have not and are asleep on the job.

"The public must have confidence that the food they buy is properly labelled, legal and safe to eat whether it is purchased from a supermarket or in a school canteen."